According to the Google Geo Developers blog the primary motivation behind this new version was speed, especially for rendering maps on mobile browsers.

Changes for v3 according to the blog include:

Chrome and iPhone Safari mobile added to our supported browsers.

Your mashups will also work on Android-based phones with the recent update, but you may notice some issues, like the “View/Save Image” dialog showing unexpectedly. We’re working with the Android team to fix this and improve the end user’s experience in interacting with the map. We could’ve waited until it’s perfect, but we really wanted to get an early release in your hands and start getting feedback while we fix up a few remaining issues.

No keys required. You can now copy ‘n paste code easily or embed in RSS readers, for example, without getting key errors.

MVC-based architecture. This allowed us to significantly reduce the size of our initial JavaScript download. We found it to be simple and powerful.

Default UI is enabled automatically. We’ll provide default UI controls and behavior (and we’ll update them) so your mashup can keep up with the latest and greatest changes we make to Google Maps. Of course, if you’ve got customized controls you’re happy with, you can disable the default UI updates.

Namespaces. Everything is always in the google.maps.* namespace and there is no “G” prefixed variables in the global scope.

Geocoding API has been overhauled based on the feedback we’ve received with the existing implementation over the past three years